1. Field of the Invention
The present invention relates to a method and a corresponding device for detecting blocking artifacts in digital video pictures.
The present invention also relates to a method and a corresponding device for processing a sequence of digital video pictures comprising a detection step of blocking artifacts and a post-processing step.
The present invention further relates to a set-top-box and a television set comprising such devices.
2. Description of the Related Art
Video sequences encoded with existing international video encoding standard can sometimes present some degradations, such as blocking artifacts. The commonly encountered degradations can go from very little impairments to heavy degradation depending on the encoding bit rate. Several methods of measuring the blocking artifact level have already been introduced. Based on the human visual sensitivity, said methods require both the original and the reconstructed images and are rather complex to implement. As a consequence, they cannot be used when the original pictures are not available.
To solve this problem, a new method is disclosed in the paper xe2x80x9cQuantitative quality metrics for video coding blocking artifactsxe2x80x9d by H. R. Wu and M. Yuen in Proceedings of Picture Coding Symposium, vol. 1, pp. 23-28, March 1996. This method uses only the reconstructed video pictures to determine a block level metric. Unfortunately, the block level metric calculation is very complex in terms of number of operations and of memory requirements, making it unrealistic for an implementation in a commercial product. Moreover, this method assumes that the first encoding block starts at the top right pixel of the picture, which is not always true if said picture has been converted to analog before being converted to digital.
It is an object of the invention to provide a method of detecting blocking artifacts contained in digital video pictures, which processes video pictures without an a prior knowledge of the original pictures or any information related to the encoding process, and which can be easily implemented in a hardware application.
To this end, the method according to the invention is characterized in that the method comprises the steps:
filtering a digital input signal using a gradient filter for providing at least one filtered signal, and
calculating a block level metric indicating if the picture has been encoded or not using a block-based processing, for processing the filtered signal(s) to identify and count blocking artifacts as a function of their position in a grid.
Such a method can detect blocking artifacts with an efficient and simple algorithm that only needs the reconstructed pictures. If the computed block level metric is lower than a threshold, the picture has either not been encoded using a block-based processing, or has been encoded in a seamless way. In the opposite case, the picture has been encoded using a block-based processing in a non-seamless way.
The method according to the invention is also characterized in that the calculation step comprises a sub-step of determining a shift of an origin of the grid in the picture in order to compute the block level metric.
Said method does not assume that the first encoding block starts at the top right pixel of the digital video picture. As a consequence, said method can be implemented directly in a television set, without knowing in advance if the incoming picture has been previously converted from digital to analog and then to digital again.
It is another object of the invention to provide a method of processing a sequence of digital video pictures comprising this step of detecting blocking artifacts and a step of post-processing the digital video pictures if the block level metric provided by the detection step is higher than a threshold.
Such a processing method benefits from the block level metric computed in the detection step in order to take the right corrective actions and, consequently, to adapt in a suitable way the post-processing step.
Finally, it is an object of the invention to provide a device implementing such a detection method. Such a device will be advantageously integrated into set-top-boxes or into up-market television sets.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.